The present invention relates to touch screen devices and to classifying touch gestures entered to a touch screen. In particular, it relates to classifying a dual touch gesture input to a four-wire resistive touch screen.
Generally, the structure of a 4-wire resistive touch screen is well known. FIG. 1 illustrates a typical 4-wire resistive touch screen. The illustrated 4-wire resistive touch screen 100 has a first plastic film layer 110 (a Y layer) with a transparent resistive coating on a bottom side, and a pair of electrodes 112 at the vertical edges of the layer. Spaced apart from the Y layer is a second plastic film layer 120 (X layer) with a transparent, resistive coating on a top side, and a pair of electrodes 122 at the horizontal edges of the layer. Below the two plastic film layers 110, 120 is an LCD screen 130 that displays image content that is viewed on the touch screen device.
During a typical classical operation, a user touches a point on the screen which causes the Y layer to deflect and make contact with the X layer. The X, Y coordinates of the point of contact can be determined during two phases of operation. In a first phase, voltage is driven on a first layer (say, the Y layer) and a voltage is read from a single electrode of the second layer (the X layer). In a second phase, a voltage is driven on the second layer (the X layer), and a voltage is read from a single electrode of the first layer (Y layer). A high impedance input device is used to read voltages from the sensing layer in each phase, which minimizes voltage losses in the sensing layer. In other terms, the layer that is driven by the applied voltage may be called the “active” layer, and the sensing layer may be called the “passive” layer. Thus, both input voltages are digitized and converted to a value representing the point of contact in the coordinate axis at which the layers touch each other.
It may be desirable for a user to interact with the screen via multiple points of contact. Multi-touch cases present additional challenges. When multiple touches are performed, voltage losses can occur in the sensing layer and, therefore, the voltages present at electrodes of the sensing layer no longer directly represent the point of contact in the driving layer.
Some touch screen systems have attempted to detect dual positions of contact. The X and Y layers in other systems may be provided with complex conductive patterns that are expensive to manufacture. There is a need for another less complex and expensive method and system for classifying gestures performed on conventional 4 wire resistive touch screens.